1. Field of the Invention
This invention relates to a discharge-lamp drive apparatus. This invention also relates to an apparatus for driving a metal halide lamp via a piezoelectric transformer. This invention further relates to power-feed control after a discharge lamp starts lighting.
2. Description of the Related Art
A typical drive apparatus for a discharge lamp includes a boosting transformer. When the lamp is required to start lighting, the boosting transformer is used to generate a starting voltage. The starting voltage is applied to the lamp so that the lamp starts lighting. After the start, power is fed to the lamp via another circuit. The typical drive apparatus has the problem that the boosting transformer is large in size.
Japanese patent application publication number 11-97758 discloses an apparatus in which a cold-cathode fluorescent lamp is activated by power fed via a piezoelectric transformer. In the apparatus of Japanese application 11-97758, the output terminal of a variable-frequency oscillation circuit is connected to the primary side of the piezoelectric transformer through a waveform shaping circuit and a drive circuit. The secondary side of the piezoelectric transformer is connected to the lamp. A start control circuit and an oscillation control circuit are connected to the variable-frequency oscillation circuit. At the start of the activation of the lamp, the start control circuit equalizes the frequency of oscillation of the variable-frequency oscillation circuit to about the resonant frequency of the piezoelectric transformer. After the lamp starts lighting, the operation of the start control circuit is suspended and instead the oscillation control circuit adjusts the frequency of oscillation of the variable-frequency oscillation circuit to maintain the current flowing through the lamp at approximately a constant level. Specifically, a detection resistor is connected in series with the lamp. The detection resistor senses the current flowing through the lamp. Information of the sensed current is fed back to the oscillation control circuit. The frequency adjustment by the oscillation control circuit is responsive to the sensed current.
The apparatus of Japanese application 11-97758 has the problem that after the start, the lamp takes a relatively long time until falling into a stably lighting state.
It is an object of this invention to provide a discharge-lamp drive apparatus which can bring the lamp into a stably lighting state in a relatively short time after the start thereof.
A first aspect of this invention provides an apparatus for driving a discharge lamp. The apparatus comprises a piezoelectric transformer connected to the discharge lamp; drive means for feeding controllable power to the discharge lamp via the piezoelectric transformer to controllably drive the discharge lamp; lighting control means for controlling the drive means to light the discharge lamp, the lighting control means including means for controlling the drive means to feed a first current to the discharge lamp during a build-up time interval before the discharge lamp changes to a stably lighting state; and means for controlling the drive means to feed a second current to the discharge lamp after the build-up time interval; wherein the first current is greater than the second current.
A second aspect of this invention is based on the first aspect thereof, and provides an apparatus wherein the lighting control means comprises impedance detecting means for detecting an impedance of the discharge lamp; and drive-state changing means for determining that the build-up time interval ends when the impedance detected by the impedance detecting means exceeds a predetermined value, and for controlling the drive means to change the discharge lamp from a drive state for the build-up time interval to a drive state for a stably-lighting time interval when determining that the build-up time interval ends.
A third aspect of this invention is based on the first aspect thereof, and provides an apparatus wherein the lighting control means comprises means for controlling the drive means to feed a third current to the discharge lamp during a former stage of the build-up time interval; and means for controlling the drive means to feed a fourth current to the discharge lamp during a latter stage of the build-up time interval, the third current being smaller than the fourth current.
A fourth aspect of this invention is based on the third aspect thereof, and provides an apparatus wherein the lighting control means comprises impedance detecting means for detecting an impedance of the discharge lamp, and drive-state changing means for determining that the former stage of the build-up time interval is replaced by the latter stage of the build-up time interval when the impedance detected by the impedance detecting means drops below a predetermined value, and for controlling the drive means to change the discharge lamp from a drive state for the former stage of the build-up time interval to a drive state for the latter stage of the build-up time interval when determining that the former stage of the build-up time interval is replaced by the latter stage of the build-up time interval.
A fifth aspect of this invention is based on the first aspect thereof, and provides an apparatus wherein the lighting control means comprises means for controlling the drive means so that during the build-up time interval, the power fed to the discharge lamp will be decreased toward a power value occurring after the build-up time interval.
A sixth aspect of this invention is based on the first aspect thereof, and provides an apparatus wherein the drive means operates at a variable drive frequency.
A seventh aspect of this invention is based on the sixth aspect thereof, and provides an apparatus further comprising means for controlling the drive frequency at which the drive means operates to adjust an intensity of light emitted from the discharge lamp in response to a light-intensity adjustment signal.
An eighth aspect of this invention is based on the sixth aspect thereof, and provides an apparatus wherein the drive frequency at which the drive means operates is higher than a resonant frequency of the piezoelectric transformer.
A ninth aspect of this invention is based on the first aspect thereof, and provides an apparatus wherein the drive means operates at a variable voltage.
A tenth aspect of this invention provides an apparatus for driving a discharge lamp. The apparatus comprises a piezoelectric transformer connected to the discharge lamp; first means for lighting the discharge lamp; second means for feeding a first power to the discharge lamp via the piezoelectric transformer after the discharge lamp is lighted by the first means; third means for detecting an impedance of the discharge lamp; fourth means for determining whether or not the impedance detected by the third means exceeds a predetermined value after the discharge lamp is lighted by the first means; and fifth means for feeding a second power to the discharge lamp via the piezoelectric transformer after the fourth means determines that the impedance exceeds the predetermined value, the second power being smaller than the first power.
An eleventh aspect of this invention is based on the tenth aspect thereof, and provides an apparatus further comprising sixth means for decreasing the first power fed to the discharge lamp during a time interval after the discharge lamp is lighted and before the fourth means determines that the impedance exceeds the predetermined value.
A twelfth aspect of this invention provides a method of driving a discharge lamp via a piezoelectric transformer. The method comprises the steps of lighting the discharge lamp; feeding a first power to the discharge lamp via the piezoelectric transformer after the discharge lamp is lighted; detecting an impedance of the discharge lamp; determining whether or not the detected impedance exceeds a predetermined value after the discharge lamp is lighted; and feeding a second power to the discharge lamp via the piezoelectric transformer after it is determined that the impedance exceeds the predetermined value, the second power being smaller than the first power.
A thirteenth aspect of this invention is based on the twelfth aspect thereof, and provides a method further comprising the step of decreasing the first power fed to the discharge lamp during a time interval after the discharge lamp is lighted and before it is determined that the impedance exceeds the predetermined value.